A radioimmunoassay (RIA) for human high molecular weight kininogen (HMWK) has been improved and more firmly established. Extensive cross-reactivity studies have been performed. The assay has been applied to 17 normal plasmas and showed excellent correlation with bioassay. Gel filtration of normal plasma showed no antigenic fragments. Seven plasmas reportedly deficient in HMWK also have been studied using the RIA. Antigen has been detected in urine.